Closed-loop purging system for laser

ABSTRACT

A method of minimizing contamination of optical components of a laser resonator is disclosed. The resonator components are located in an enclosure, which may contain contaminants including water vapor and organic favor released by the optical components, mounts of the optical components, or the enclosure itself. The enclosure may also contain suspended particulate matter. In order to reduce the level of these contaminants, a purging system extracts gas from the enclosure and passes the gas through a desiccant, an organic vapor trapping material, and a particulate matter filter then returns the extracted gas to the enclosure. The purging system is particularly useful for ultrafast lasers and ultraviolet lasers where the power of the laser radiation increases the probability of destabilizing reactions between laser radiation and contaminants.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally hermetic sealing of lasers. Theinvention relates in particular to a closed-loop purging system forwater vapor, organic vapor and particulate content in an enclosuresurrounding an ultrafast laser resonator or an ultraviolet (UV) laserresonator.

DISCUSSION OF BACKGROUND ART

Ultrafast lasers are generally regarded as being lasers that deliveroutput radiation in pulses having a duration of a few hundredfemtoseconds or less. One common ultrafast laser is a Ti:sapphire laser,which can be arranged to deliver output radiation at wavelengths betweenabout 700 nanometers (nm) and about 1000 nm. The pulses delivered oftenhave a relatively low energy, for example, tens of millijoules (mJ) toas little as tens of nanojoules (nJ). The short pulse-duration can causethe pulses to have a very high peak power, for example, on the order ofgigawatts per square centimeter (GW/cm²) in certain locations in aresonator.

The very high peak powers delivered by such lasers can rapidly causedamage to optical components of the lasers, absent measures to inhibitsuch damage. Laser damage to optical components may be exacerbated bydefects on or in optical surfaces of the components. Accordingly, it isnot unusual that at least some portion of the optical components of anultrafast laser are generated by so-called super-polishing techniqueswhich yield surfaces having a surface smoothness of atomic dimensions,for example, about 4 Ångstrom Units (Å) root-mean-square (RMS) or less.Optical coatings for such super-polished components, reflective coatingsin particular, are often deposited by ion-beam sputtering (IBS). IBS isa coating deposition method that can provide coatings having a highdegree of chemical perfection and very low defect content. Thisminimizes absorption and scattering of radiation by the coatings.However, a super-polished, IBS-coated optical component can be as muchas about five or more times more expensive than a similar componentpolished and coated by more conventional methods. Such additionalexpense can be wasted if the components are later contaminated byparticulate matter, condensates, vapors, or the like.

It is not unusual in commercial laser manufacture to assemble lasers inclean-room conditions to minimize particulate deposition on opticalcomponents of the lasers. In such a case, it would be usual to place atleast the optical resonator of the laser in an enclosure sufficientlysealed to minimize at least ingress of particulate contaminants, andpreferably also, ingress of contaminants in gaseous or vapor form. Suchan enclosure may be purged, before sealing, with filtered dry nitrogen,dry air or the like.

By implementing one or more above-discussed measures duringmanufacturing and assembly, an ultrafast laser may be operated for atotal as long as several thousand before the performance of the laserbecomes significantly diminished by laser damage to one or more opticalcomponents thereof. It is believed, however, even if an enclosure couldbe perfectly hermetically-sealed, damage to optical components mayresult from contamination of optical components by outgassing productsof the optical components, adhesives and the enclosure itself.Outgassing products can be generated while the laser is operating andalso while the laser is not operating.

It is believed that the most problematical of the outgassing productsare organic vapors, which can be released from material such asadhesives, elastomer seals, and any plastic materials used in theconstruction of the enclosure. Water vapor may also be released fromcomponents of the enclosure or optics therein. The water vapor and theorganic vapors can condense directly on surfaces of the opticalcomponents. The water vapor and organic vapors together or incombination can react with laser radiation while the laser is operating.Products of the reactions can also condense or be deposited on theoptical surfaces. These reaction products may include particulate mattersuch as carbon particles or soot. Most of these reaction products, ifcondensed or deposited on the optical surfaces can increase thevulnerability of the optical surfaces to damage by the laser radiation.Even if reaction products were only present within the atmosphere of theenclosure this could still result in unstable operation of the laser.

SUMMARY OF THE INVENTION

The present invention is directed to a method of minimizingcontamination of optical components of a laser, the components beinglocated in a gaseous atmosphere within an enclosure. The gaseousatmosphere can contain contaminants including water vapor, organicvapor, and suspended particulate matter. These contaminants may bepresent at some low level, for example, hundreds of parts per billion orless, immediately after the components are placed in the enclosure. Thecontaminant level can increase with both operational and non-operationaltime of the laser.

In one aspect of the present invention, the method comprises extractinggas from the atmosphere within the enclosure. The extracted gas ispassed through a first medium selected to reduce the water vapor contentof the extracted gas; through a second medium selected to reduce theorganic vapor content of the extracted gas; and through a filterselected to reduce the particulate matter content of the extracted gas.After the extracted gas is passed through the first and second media andthe filter, it is returned to the enclosure.

The extraction and replacement cycle preferably takes place continuouslyduring operation of the laser such that the water vapor, organic vapor,and particulate matter content of the atmosphere in the enclosure ismaintained at a minimum consistent with the selection of the media andthe filter.

In another aspect of the invention, apparatus for carrying out themethod includes a gas conditioning arrangement including the first (adesiccant) medium, the second (a medium for trapping organic vapors)medium, and the filter for trapping particulate matter. The apparatusincludes a pump, which is arranged to extract gas from the enclosure anddeliver the extracted gas to the gas-conditioning arrangement. The gasconditioning arrangement is configured such that the extracted airdelivered thereto by the pump passes through the desiccant medium, theorganic vapor trapping medium, and the filter, and is then returned tothe enclosure.

In one preferred embodiment, the apparatus further includes first andsecond valves. The first and second valves are arranged such that adrying gas may be circulated through the desiccant medium forregenerating the desiccant medium while preventing the drying gas fromreaching the enclosure.

Maintaining a low organic vapor content in a laser resonator isparticularly important if the laser resonator is an ultrafast laserresonator or a laser resonator arranged to generate ultraviolet laserradiation. The relatively high-energy of ultraviolet laser radiation,multiphoton processes in the case of ultrafast lasers, generating longerwavelength radiation can increase the probability of reactions betweenthe laser radiation and the organic vapors or their condensates. Asnoted above, products of these reactions, including particulate matter,can lead to unstable operation of the laser, or accelerated damage tooptical components of the laser resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an ultrafast laser including anultrafast laser resonator, a source of optical pump light, a controller,and a purging system in accordance with the present invention.

FIG. 2 schematically illustrates details of the laser resonator of FIG.1, the laser resonator being located in an enclosure cooperative withthe purging system of FIG. 1.

FIG. 3 schematically illustrates one preferred embodiment of the purgingsystem of FIG. 1.

FIG. 4 schematically illustrates another preferred embodiment of thepurging system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, laser 20 includes a laser resonator 22, asource 24 of optical pump light, and a purging system 26 in accordancewith the present invention. In this example, laser resonator 22 is anultrafast laser resonator delivering laser radiation in the form ofultrafast output pulses 28. Laser 20 also includes a controller 30arranged to control operations and parameters the laser resonator, thepump light source, and the purging system. Controller 30 controlsoperations of purging system 26 via electrical connections 32, 34, and36. The controller controls operations and parameters of laser resonator22 via electrical connections 38, 40, and 42, and controls pump lightsource 24 via electrical connection 43. Purging system 26 is cooperativewith laser resonator 22 via conduits 44 and 46. Purging system 26 alsoincludes conduits 48 and 50, which connect with a desiccant module (notshown in FIG. 1) in the purging system. The function of conduits 48 and50 is described in detail further to hereinbelow.

Referring now to FIG. 2, laser resonator 22 includes a resonant cavity52 terminated by mirrors 54 and 56. Mirror 54 is a maximum reflectingmirror. The inclination of mirror 54 can be adjusted by controller 30via electrical connection 42 and a mirror mount 57 including actuators58. Mirror 56 is a partially transmitting mirror, which allows outputpulses 28 to be delivered from the resonant cavity. A portion of theoutput pulses is sampled by a beamsplitter 62 and detected by a detector64. Output of detector 64 is connected to controller 30 by connection 38for use by the controller in controlling parameters of the laserresonator. The optical path of resonant cavity 54 is folded by foldmirrors 66, 68, 70, and 72. Folding of the optical path reduces thephysical length of the resonant cavity.

Resonant cavity 54 includes a gain medium 74 located between mirrors 66and 70. In this example the gain medium is Ti:sapphire, which providesoptical gain in a wavelength region between about 700 and 1000nanometers (nm). Pump light source 24, in this example, is afrequency-doubled Nd:YVO₄ laser, delivering pump light by fold mirrors awavelength of 532 nm. Pump light from source 24 is delivered to gainmedium 74 through mirror 70. Also located in resonant cavity 52 are twoprisms 76 and 78. The prisms are arranged to compensate for group delaydispersion of laser radiation circulating in resonant cavity 52, and arealso used to tune the output wavelength of the laser resonator.

Prism 78 is mounted on a movable carrier 80, the movement of which iscontrolled by controller 30 via electrical connection 40. A slit 81defines a portion of prism 78 through which optical radiation can pass.The output wavelength of pulses 28 is changed or tuned by operatingcarrier 80 such that prism 78 is moved to a new location, indicated inFIG. 2 by dotted triangle 78A. Slit 81 is moved synchronously with theprism as indicated by line 81A. Dotted lines 82 indicate a change inoptical path in the resonator resulting from the movement of prism 78.Laser resonator 22, in this example, is a mode locked laser resonator.Mode locking of the laser resonator is effected by an aperture 84located in resonant cavity 52 and cooperative with a Kerr-lens effectinduced in gain medium 74 by pump light delivered from pump-light source(laser) 24.

A detailed explanation of operating principles of resonant cavity 52 isnot required for understanding principles of the present invention.Accordingly, such an explanation is not presented herein. A detailedexplanation of an ultrafast laser including a resonant cavity similar toresonant cavity 52 is provided in co-pending application Ser. No.09/813,507 the complete disclosure of which is hereby incorporated byreference.

Continuing now with reference to FIG. 2, optical components of laserresonator 22 are located in an enclosure indicated in FIG. 2 by dottedline 90. Pump light from laser 24 enters enclosure 90 via a window 92.Laser output pulses 28 leave the enclosure via a window 94. Othergeneral construction principles of an enclosure such as enclosure 90 arewell known to those skilled in the art to which the present inventionpertains. Accordingly, such principles are not described or depictedherein. A feature of enclosure 90 specific to the present invention,however, is the connection of the enclosure to conduits 44 and 46, whichprovide fluid communication between the enclosure and components ofpurging system 26.

The interior (atmosphere) 90A of enclosure 90 is maintained at aboutambient atmospheric pressure. The atmosphere of enclosure 90 willusually be an air atmosphere. If enclosure 90 is sufficiently wellsealed, however, an atmosphere of nitrogen or some other inert gas maybe included. Whatever the gaseous atmosphere of enclosure 90, it can beexpected to include some finite level of contaminants, however smallthat level. As discussed above, these contaminants may include watervapor, organic vapors, and particulate matter. As noted above,particulate matter may include that which was present at the time thatthe enclosure was closed, and particulate matter generated as a resultof interaction between laser radiation circulating in resonant cavity 52and the organic vapors.

Referring now to FIG. 3, one preferred embodiment 26A of a purgingsystem 26 in accordance with the present invention includes a pump 102and a gas conditioning arrangement 104. Gas conditioning arrangement 104includes a container 106 containing a desiccant material 108. Desiccantmaterial 108 is preferably silica gel, but maybe any desiccant material.Gas conditioning arrangement 104 also includes a container 110 includingan organic vapor trapping material 112. A preferred organic vaportrapping material is a high surface-area coconut-shell based activatedcarbon. Organic vapor traps including this material are available invarious sizes from Agilent Technologies, Inc. of Palo Alto, Calif. Othersuitable organic vapor trapping materials include a 5 Å molecular sieve.

A filter unit 114 is provided for filtering particulate matter. Filterunit 114 is preferably capable of trapping particles having a size ofabout 0.5 micrometers (μm) and greater, for example, a HEPA filter. Onesuitable HEPA filter is available from the Pall Gellman Sciences Inc. ofAnn Arbor, Mich. as HEPA Capsule Part No. 12144. This filter has a poresize of 0.3 μm and has a filtering efficiency of 99.97% for 0.3 μm DOPaerosol.

Pump 102 extracts gas from the atmosphere of enclosure 90 via conduit44. The pump delivers the extracted gas via a conduit 120 and a two-wayvalve 122 to gas conditioning arrangement 104. The circulation directionof gas through the purging system is indicated in FIG. 3 by arrows A.The gas delivered by pump 102 is urged by the pump through the desiccantmaterial (medium) 108; through a conduit 124; through another two-wayvalve 126; and then through organic vapor trapping material 112. Afterpassing through the organic vapor trapping material, the gas passesthrough HEPA filter 114 into conduit 46, which returns the gas toenclosure 90. As noted above, desiccant material 118 reduces the watervapor content of the gas, and organic vapor trapping material 112reduces the organic vapor content of the gas. HEPA filter 114 reducesthe particulate matter content of the gas. Valves 122 and 126 in thismode of operation prevent any of the extracted gas from escaping thepurging system via conduits 48 and 50.

In one preferred cycle of operation of purging system 26A, theextraction and return of gas from and to the enclosure takes placecontinually during any period in which laser 20 is operating. Operationof the purging system is started and stopped by correspondingly startingor stopping pump 102 by commands delivered thereto from controller 30via electrical connection 32. Continuous operation of purging system 26Acan provide that in the atmosphere of enclosure 90, the water vapor,organic vapor, and particulate matter content of the atmosphere aremaintained at minimum consistent with the materials and configuration ofgas conditioning arrangement 104. It is possible, of course, that theinventive purging system could be activated and deactivated bycontroller 30 based on measurements of particle count or concentrationsof particular vapor species. This, however, would require providingcorresponding sensors, which could increase the cost of a laser or thepurging system.

After a period of operation, depending on the ambient atmosphere inwhich laser 20 is located, or the conditions of operation of the laser,desiccant material 108 may become saturated with water vapor. Shouldthis occur, desiccant material 108 may be revived or regenerated bypassing a drying gas, such as dry air or dry nitrogen, through thematerial, as follows.

Operation of pump 102 is stopped. Valve 122 is switched to prevent airfrom being delivered from pump 102 to the desiccant material, and toallow the drying gas to be delivered to the desiccant material viaconduit 48. Valve 126 is switched to prevent any drying gas fromreaching enclosure 90 via the organic vapor trapping material, the HEPAfilter, and conduit 46. This switching allows drying gas delivered tothe desiccant material via conduit 48 to pass through the desiccantmaterial and exit the purging system via conduit 50 as indicated in FIG.3 by arrows D. After the desiccant material has been regenerated, valves122 and 126 are switched back to a position that allows gas extractedfrom enclosure 90 to pass to the gas conditioning system and return tothe enclosure via conduit 46. Valves 122 and 126 may be operated bycommands delivered thereto along connections 34 and 36 from controller30.

Another preferred embodiment 26B of a purging system in accordance withthe present invention is depicted in FIG. 4. Purging system 26B issimilar to purging system 26A of FIG. 3 with an exception that desiccantmaterial 108 and organic vapor trapping material 112 are contained in asingle container 130. A permeable diaphragm or separator 132 separatesthe desiccant material from the organic vapor trapping material. Onesuch combined desiccant and organic vapor trapping unit is availablefrom the W. A. Hammond Drierite Company Ltd of Xenia, Ohio as PartNo.27068.

It is emphasized here that the sequence of vapor reduction and filteringis particularly important in the method and apparatus of the presentinvention. If water vapor reduction does not precede organic vaporreduction there could be a significant degradation in the efficiency oforganic vapor reduction. As there is a possibility that water vaporremoval materials and organic vapor trapping materials can generateparticulate matter it is important that particulate matter filteringtakes place following water vapor reduction and organic vapor reduction.

In the description of laser 20 given above, laser resonator 22,controller 30, and purging system 26 are described as separate units.This arrangement should not be construed as limiting the presentinvention. By way of example, as the size of the purging system can berelatively small compared with the laser resonator, the purging systemand the laser resonator may be combined in a single unit or housing.Alternatively, the purging system may be combined in a single housingwith the controller. In another arrangement, purging system 26 may beconfigured as a stand-alone module including a dedicated controllerseparate from controller 30. One skilled in the art to which the presentinvention pertains may devise other configurations of the purgingsystem, the laser resonator and one or more controllers withoutdeparting from the spirit and scope of the present invention.

The purging system of present invention is described above withreference to its use with a Ti:sapphire ultrafast laser. This should notbe construed as limiting the present invention. The inventive purgingsystem is applicable to other ultrafast lasers such as those includingdyes or semiconductor materials as gain media. As noted above, the veryhigh-power and short duration of ultrafast laser pulses can increase thepossibility of the ultrafast laser radiation reacting with any organiccontaminants that may be found in the atmosphere in the resonant cavityof the laser. Also as noted above, the inventive purging system isparticularly useful in ultraviolet lasers where the high-energy of theultraviolet radiation also increases the possibility of reactions withany organic contaminants in the laser resonator. Costs permitting,however, it may be found useful to use the inventive purging system withany other laser with the goal of extending the operating lifetime ofoptical components or reliability of operation of the laser.

The present invention is described above in terms of a preferred andother embodiments. The invention is not limited, however, to theembodiments described and depicted herein. Rather, the invention islimited only by the claims appended hereto.

What is claimed is:
 1. A mode-locking laser system, comprising: a sealedenclosure; a solid state gain medium located in the sealed enclosure; apump source for optically pumping the solid state gain medium in orderto produce optical radiation; a plurality of optical components locatedin a gaseous atmosphere within said enclosure and defining a resonantcavity for said optical radiation, said optical components releasingwater vapor and organic vapors into said gaseous atmosphere, and withparticulate matter being generated by interaction of said opticalradiation with one or more of said water vapor and said organic vapors;a gas conditioning arrangement including a desiccant medium, a mediumfor trapping organic vapors, and a filter for trapping particulatematter; a pump, said pump in fluid communication with said enclosure viaa first conduit and in fluid communication with said gas conditioningarrangement via a second conduit and said gas conditioning arrangementbeing in fluid communication with said enclosure via a third conduit;said pump being arranged to extract gas from said enclosure via saidfirst conduit and deliver said extracted gas to said gas-conditioningarrangement via said second conduit; and said gas conditioningarrangement being configured such that said extracted gas deliveredthereto by said pump passes, in sequence, through said desiccant medium,said organic vapor trapping medium, and said filter and is then returnedto said enclosure via said third conduit and operated only in a mannerintended to reduce water vapor, organic vapor and particulate matterfrom said gaseous atmosphere.
 2. The mode-locking laser system of claim1, wherein said desiccant medium is silica gel.
 3. The mode-lockinglaser system of claim 1, wherein said organic vapor trapping medium isactivated carbon.
 4. The mode-locking laser system of claim 1, whereinsaid organic vapor trapping medium is a molecular sieve.
 5. Themode-locking laser system of claim 1, wherein said filter is a HEPAfilter.
 6. The mode-locking laser system of claim 1, further includingfourth and fifth conduits and first and second valves, said fourth andfifth conduits and said valves arranged such that a drying gas can bepassed through said desiccant medium for regenerating the desiccantmedium while preventing said drying gas from entering said enclosure.